1. Field of the Invention
This invention relates to a switching power supply of a synchronous rectification type for rectifying a voltage induced in a secondary winding of a switching transformer thereof in synchronism with induction of the voltage.
2. Description of the Related Art
Conventional switching power supplies usually employ the diode rectification method of rectifying and smoothing an alternating current output from a secondary winding of a switching transformer thereof by using a diode and a capacitor arranged on the side of the secondary winding. On the other hand, in recent years, a synchronous rectifier circuit using an FET as a rectifying element has been actively developed with a view to reducing power loss caused by the diode during the rectification. A power supply 71 shown in FIG. 8 is an example of the related art, i.e. the switching power supply having a synchronous rectifier circuit of the above-mentioned type.
The power supply 71 is basically a flyback switching power supply including a synchronous rectifier circuit disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 9-312972 filed by the present assignee. More specifically, the power supply 71 includes a switching transformer 2, and a primary circuit (primary winding-side circuit) on the side of a primary winding 2a of the transformer 2, which is comprised of a diode stack 11 for rectifying an alternating current output from an AC power source PS, a smoothing capacitor 12, a MOS field effect switching transistor (hereinafter referred to as "the FET") 14, a resistance 16 of a bias circuit, and a switching control circuit 17 for controlling switching operation of the FET by a frequency control method or a PWM (Pulse Width Modulation) control method. In this primary circuit, a capacitor 15 shown in the figure is implemented by a capacitance between the source and drain of the FET 14, or a capacitor arranged separately from the capacitance of the capacitor 15.
A synchronous rectifier circuit 72 is arranged in a secondary circuit (secondary winding side-circuit) on the side of a secondary winding 2b of the transformer 2. The synchronous rectifier circuit 72 is comprised of a current transformer 21, an FET 22, resistances 23, 24 of a bias circuit, a diode 25, and a smoothing capacitor 26. The current transformer 21 has a primary winding 21a connected in series with an output line for outputting a rectified current to an external load, and a secondary winding 21b having the number of turns n times as large as that of turns of the primary winding 21a (i.e. turn ratio of the secondary winding 21b to the primary winding 21a is equal to n) and serving as a current pickup winding. From the secondary winding 21b, the current transformer 21 outputs a control current I.sub.12 having a current value which is equal to the current value of a current I.sub.11 flowing through the primary winding 21a multiplied by the reciprocal (1/n) of the turn ratio n.
The FET 22 includes an inner parasitic diode 27. When an alternating current induced in the secondary winding 2b of the transformer 2 flows in the same direction as that of a voltage V.sub.S11 indicated in FIG 8, the FET 22 permits positive part of the alternating current to pass therethrough via the inner parasitic diode 27, whereas when the alternating current induced in the secondary winding 2b is directed in the same direction as that of a voltage V.sub.S12 indicated in FIG. 8, the FET 22 prevents the alternating current from passing therethrough.
In the power supply 71, when an alternating current is output from the AC power source PS, the alternating current is rectified to a pulsating current by the diode stack 11, and the pulsating current is smoothed to a DC current by the capacitor 12. Then, the DC current is switched by the FET 14 under the control of the switching control circuit 17, whereby a current I.sub.D (see a left side portion of FIG. 9A) flows into the primary winding 2a of the transformer 2 to accumulate energy in the transformer 2. Next, when the FET 14 is switched off, the current I.sub.11 (see a left side portion of FIG. 9B) is caused to be output from the secondary winding 2b by the energy accumulated in the transformer 2. In this case, the current I.sub.11 flows through a closed loop of the secondary winding 2b of the transformer 2, the primary winding 21a of the current transformer 21, the capacitor 26, and the inner parasitic diode 27, whereby the current I.sub.11 is smoothed by the capacitor 26. In this state, when the current I.sub.11 passes through the primary winding 21a, the control current I.sub.12 is output from the secondary winding 21b to flow into the gate of the FET 22 via the resistance 24 to charge the gate capacitance. After the gate capacitance of the FET 22 is charged, the control current I.sub.12 flows through a closed loop of the secondary winding 21b and the resistances 24, 23, whereby a voltage V.sub.G (see a left side portion of FIG. 9C) generated across opposite ends of the resistance 23 is applied to the gate of the FET 22.
When the voltage V.sub.G applied to the gate of the FET 22 becomes higher than an ON voltage V.sub.ON of the FET 22, the FET 22 is turned on, as shown in a left side portion of FIG. 9D, to permit the current I.sub.11 to pass between the source and drain of the FET 22. As a result, the alternating current induced in the secondary winding 2b is rectified mainly by using the FET 22. In this case, the rectification causes power loss amounting to a value obtained by multiplying the square of the rectified current by the ON resistance of the FET 22, which is far smaller than power loss which would be suffered by the power supply 71 when it employs the diode rectification method.
Next, when the current I.sub.11 stops flowing, the control current I.sub.12 also stops flowing, and accordingly, the voltage V.sub.G applied to the gate of the FET 22 is lowered. In this process, the electric charge accumulated in the gate of the FET 22 is released to the low potential line via the diode 25 and the secondary winding 21b of the current transformer 21, and hence the gate voltage V.sub.G is instantly decreased to 0V, thereby causing the FET 22 to stop its operation in an extremely short turn-off time. As a result, when the current I.sub.D flows through the primary winding 2a next time, the FET 22 is maintained in a completely inoperative state, and in this state, the direction of a current about to flow in the secondary winding 2b and the forward direction of the inner parasitic diode 27 are opposite to each other, so that the current is inhibited from flowing through the secondary winding 2b, thereby reliably preventing the generation of a countercurrent which charges the capacitor 26 in the opposite direction.
As described above, according to the power supply 71, the current transformer 21 generates and outputs the control current I.sub.12 having a current value approximately proportional to a current value of the current I.sub.11 rectified by the inner parasitic diode 27, to thereby make the FET 22 operative. This enables an increased rectifying efficiency to be achieved in comparison with the diode rectification method.
However, the switching power supply 71 has room for improvement as to the following points: It is true that no particular inconveniences occur when the switching control circuit 17 controls the switching operation of the FET 14 by the frequency control method, but when the switching operation of the FET 14 is controlled by the PWM control method, as shown in a right side portion of FIG. 9A, the transformer 2 sometimes completes release of the accumulated energy before the FET 14 is turned on next time. In such a case, there occurs a so-called discontinuous current mode in which the current I.sub.11 flowing through the secondary winding 2b and a current flowing through the primary winding 2a becomes discontinuous. At this time, the capacitor 15 on the primary circuit side has been charged during the ON period of the FET 14 to a voltage V.sub.C15 represented by the following equation: EQU V.sub.C15 =V.sub.C12 +V.sub.O.multidot.N.sub.1 /N.sub.2
wherein, V.sub.C12, V.sub.O, N.sub.1 and N.sub.2 represent a voltage across opposite ends of the capacitor 12, the voltage value of the output voltage V.sub.O, the number of turns of the primary winding 2a, and the number of turns of the secondary winding 2b, respectively.
This causes a phenomenon of resonance. That is, the charged energy causes a current I.sub.13 to flow through a current path of the capacitor 15, the primary winding 2a of the transformer 2, and the capacitor 12, and inversely when the voltage across the opposite ends of the capacitor 15 becomes lower than the voltage across the opposite ends of the capacitor 12, a current directed in an opposite direction to the direction of flow of the current I.sub.13 flows through the same current path. In such a case, since the current I.sub.11 flows through the secondary winding 2b, the current I.sub.12 also flows through the secondary winding 21b of the current transformer 21. Accordingly, as shown in a right side portion of FIG. 9C, the voltage V.sub.G applied to the gate of the FET 22 becomes higher than the ON voltage V.sub.ON, so that, as shown in a right side portion of FIG. 9D, an abnormal operation of the synchronous rectifier circuit can be caused in which when the FET 22 should be controlled to an OFF state, it is intermittently turned on. In such a case, as shown in FIGS. 9A and 9D, when the FET 22 is in the ON state, if the FET 14 is simultaneously turned on, the voltage V.sub.S12 induced in the secondary winding 2b is short-circuited via the FET 22, the capacitor 26, and the primary winding 21a of the current transformer 21, which causes a large current to momentarily flow in a direction opposite to a normal direction. This can result in breakage of the FET 22 or an input fuse or a great deal of switching loss due to the large current flowing through the FET 22. To eliminate this inconvenience, the power supply 71 is demanded to be free from such an abnormal operation of the synchronous rectifier circuit.
Further, the power supply 71 uses the primary circuit of a so-called capacitor input type. Hence, an input current flows into the capacitor 12 in the form of pulses, which generates so-called input current harmonics. Therefore, when the power supply 71 has a large rated power, or when a plurality of power supplies 71 are put into operation at the same time, the harmful harmonic components of the input current leak to a commercial electric system, thereby causing the problems of harmonic interference or heating of electric power apparatus due to voltage distortion. To eliminate these inconveniences, the power supply 71 is demanded to have an improved power factor.